Features/httpinput-enhancements

@rafrombrc @trink 
- Allow for an array of URLs in HttpInput TOML configuration
- Inject Timestamp, UUID, URL as Logger
- Configuration defined defaults for severity levels
- Populate HTTP GET Metadata in Fields. (StatusCode, Status, Protocol)
- Successful HttpInput GETs default severity to 6, Unsuccessful error messages at Severity 1
- Generate internal heka message of type heka.httpdata.error with severity 1 for non-200 status code GETs and failed connections; useful for health checks.